As feature size decreases, density increases, and the size of semiconductor wafers or substrates increase, Chemical Mechanical Planarization (CMP) process requirements become more stringent. Substrate to substrate process uniformity as well as intra-substrate planarization uniformity are important issues from the standpoint of producing semiconductor products at a low cost. As the size of dies increases a flaw in one small area increasingly results in rejection of a relatively large circuit so that even small flaws have relatively large economic consequences in the semiconductor industry.
Many factors are known in the art to contribute to uniformity problems. These include distribution of a slurry between a surface of the substrate and a polishing surface during the polishing operation when there is relative motion between a polishing head on which the substrate is held and the polishing surface during the polishing operation. Slurry is a, usually, chemically active liquid having an abrasive material suspended therein that is used to enhance the rate at which material is removed from the substrate surface.
Referring to FIG. 1, a typical CMP apparatus 10 includes (i) a platen 12 having a polishing surface 14 thereon; (ii) a polishing head 16 adapted to hold a substrate 18 against the polishing surface during a polishing operation; (iii) a drive mechanism (not shown) to rotate the platen 12 providing a relative motion between the polishing head 16 and the polishing surface 14 during the polishing operation; and (iv) a dispenser (not shown) adapted to dispense a slurry on the polishing surface 14 during the polishing operation. The polishing head 16 includes a carrier 19 having a subcarrier 20 with a lower surface 22 for pressing the substrate 18 against the polishing surface 14 during the polishing operation, and a retaining ring 24 circumferentially disposed about the subcarrier. The retaining ring 24 generally restrains or limits lateral movement of the substrate 18 relative to the subcarrier 20 to hold or retain the substrate between the subcarrier and the polishing surface.
One problem with a conventional CMP apparatus 10 is a non-uniform distribution of slurry between a surface 26 of the substrate 18 and the polishing surface 14 during the polishing operation. This is a result of a substantial portion of the slurry being directed around the polishing head 16 by the retaining ring 24, rather than passing under the retaining ring into the space between the substrate surface 26 and the polishing surface 14. Moreover, the limited or reduced amount of slurry that does enter this space is usually insufficient to flush out or remove the used slurry and/or solid polishing byproducts that can build up at a trailing edge 28 of the retaining ring 24 and damage the substrate 18.
Another related problem with conventional CMP apparatuses 10 and methods, is friction induced vibration during polishing due to non-uniform slurry distribution between the substrate surface 26 and the polishing surface 14.
Accordingly, there is a need for an apparatus and method that provides a uniform distribution of slurry between the surface of the substrate and the polishing surface during a polishing operation. There is a further need for an apparatus and method capable of reducing or eliminating friction induced vibration during the polishing operation. There is a yet further need for an apparatus and method capable of removing used slurry and polishing byproducts from under the surface of the substrate during the polishing operation thereby eliminating buildup of solid polishing byproducts that can damage the substrate.